A fuel cell is a device that generates electricity by a chemical reaction. Typically, in a fuel cell, an oxygen gas, such as O2, is reduced to oxygen ions (O2−) at the cathode, and a fuel gas, such as H2, is oxidized with the oxygen ions to form water at the anode. Among various types of fuel cells, solid oxide fuel cells (SOFCs) use hard ceramic compounds of metal oxides (e.g., calcium or zirconium oxides) to form components of the fuel cell, such as, for example, the anode, cathode, electrolyte, and interconnect. Fuel cells are generally designed as stacks, whereby subassemblies, each including a cathode, an anode and a solid electrolyte between the cathode and the anode, are assembled in series by locating an electrical interconnect between the cathode of one subassembly and the anode of another.
One SOFC cell design consists of five layers. Two of these five layers are relatively thick layers: anode bulk and cathode bulk. A relatively thin layer of electrolyte and relatively thin functional anode and cathode layers are sandwiched between the bulk layers. Typically, the thin layer thickness is only about 1/100th of the bulk layer thickness. The most important mechanism in stress generation in a SOFC co-fired cell is the step of cooling down from the sintering temperature, typically 1300-1400° C., to room temperature, due to the mismatch in the coefficients of thermal expansion (CTE) between the materials of the five layers. Where there is sufficient mismatch of CTE among the layers, cooling, or any temperature change that is too rapid, can cause fracture and consequent failure of the SOFC. Because of the much larger thickness, the majority of the stress is generated by the mismatch between the CTEs of the anode bulk layer and the cathode bulk layer. Since most ceramics show linear elastic stress-strain behavior up to failure in the temperature range that a SOFC operates, for a fixed geometry design, there are only two material properties that affect the thermal mismatch stress: modulus and coefficient of thermal expansion of the materials.
In order to reduce the thermal mismatch stress, it is desirable to have both the cathode and anode materials of a SOFC have a CTE that is as close as possible to the CTE of the electrolyte, which is typically made of yttria-stabilized zirconia (YSZ). A typical CTE of YSZ generally is in a range of between about 10.5×10−6° C.−1 and 11×10−6° C.−1, which is much lower than the CTE of most cathodes. Specifically, while a typical anode material has a CTE of 11.3×10−6° C.−1, the most commonly used cathode material for SOFCs is lanthanum strontium manganite (LSM), La0.8Sr0.2MnO3 (LSM20/80), which has a CTE in a range of between about 12.2×10−6° C.−1 and about 12.4×10−6° C.−1 (average CTE between room temperature and 1200° C.). See L. Kindermann, et al., Synthesis and properties of La—Sr—Mn—Fe—O perovskites, Proceedings of the 3rd European solid oxide fuel cell forum, 1998, pp. 123. The difference between the CTEs of the LSM and YSZ materials would generate a large thermal mismatch stress in the SOFC. With a Sr content increase above 0.2, the CTE of the LSM material would further increase. Id. On the other hand, it is often undesirable to use an LSM with Sr content lower than 0.2, due to diminished electrochemical performance.
Therefore, there is a need to overcome or minimize the above-mentioned problems.